SC Ratings 2

[tulum]

Hello,

I am pricing out some switchgear, and I am having some problems with the ratings...

One of the non-fused switches is rated as follows:

momentary: 40ka Asym (25ka Sym)
Fault Close Current:40ka Asym (25ka Sym)
Short time Current@ 2 sec:40ka Asym (25ka Sym)

Can all these ratings be the same?? and isn't momentary equal to Short time as defined by ANSI?

I thought if;
RMS Sym = 25ka then;
momentary = 1.6*25 = 40ka = short time, and;
Peak Short time withstand = 2.6*25 = 65kA

Furthermore, how do these relate to the nomenclature I have heard people use such as Isc, Ith and Ip?

Maybe a FAQ on SC ratings - IEC and ANSI would help??!

THanks

 

[dpc]

Not sure what voltage you are talking about. Each type of gear has its own test standard.

But momentary is not the same as short time. Momentary is the maximum rms current occuring in the first half-cycle of the fault. The multiplier for asymmetrical current depends on the power factor specified in the test standard. For ANSI medium-voltage switchgear, "momentary" is the same as the close and latch rating.

Short-time rating is a withstand rating indicating maximum current the switch can withstand for the specified time without flying apart or melting down. The duration depends on the equipment standards.

 

[jbartos]

Suggestion: Reference ANSI C37.06-1979
has two basic categories of preferred ratings for medium voltage circuit breakers, namely: Rated Values and Related Required Capabilities

Rated Values:
Rated continuous current at 60Hz, rms
Rated short circuit current (at rated max kV) kA, rms

Rated Required Capabilities:
Max Symmetrical Interrupting Capability kA, rms
3-seconds short time current carrying capability kA, rms
Closing and Latching capability 1.6K times rated short circuit current kA, rms
If expressed in peak amperes, the close and latch value is equal to 2.7K times rated short circuit current.

K is the rated voltage range factor, which is a ratio of rated maximum voltage to the lower limit of the range of operating voltage in which the required symmetrical and asymmetrical current interrupting capability vary in inverse proportion to the operating voltage.

 

[tulum]

Thank you DPC and Jbartos,

So how does "interrupting" current fit into the picture?

Another tender I have seen for switchgear says that the gear "shall have a Interrupting Capacity of 35kA". However, later on in the spec it says that the same gear "shall withstand a fault level 1/2 cycle, 35kA RMS symmetrical".

Are these two values the same?? From the above f I assume interupting capability = Momentary = close and latch, then this is true...


Regards,
TULUM

 

[dpc]

For medium-voltage breakers, the interrupting rating will be lower than the "close & latch" rating. Interrupting rating is symmetrical rms current that the breaker can safely interrupt across its opening contacts. The fault current peaks in the first half-cycle and then decays. The amount and rate of decay is dependent on several factors, including system X/R ratio and the decrement curves of local generators. The close & latch rating is the peak current that the breaker is rated to withstand, and more importantly, be able to close and latch into. In ANSI standards, the close & latch rating is generally stated in peak asymmetrical amps, rms. The interrupting rating is generally stated in symmetrical rms amps.

Per ANSI requirements, the close & latch rating must be at least 1.6 times the interrupting current.

It is easier for the gear to withstand the mechanical forces of fault current (the close and latch rating)than it is to actually interrupt the flow of current.

IEC terminology is a little different, but the physics doesn't change.